1. Field of the Invention
The present invention relates generally to power switching circuits having a transformer-coupled gate control, and more specifically, to a gate drive control circuit using a control signal modulated to a rate higher than the switching rate.
2. Background of the Invention
Transformer coupling of gate drive control is used in power switching circuits in which the transformer gate control signal either requires complete DC isolation from the switching control circuit, or in which the gate control voltage for at least one of the switching transistors is sufficiently high with respect to the controller integrated circuit operating voltage that transformer coupling of the gate control signal relaxes the voltage-handling requirements for the control circuit drive output(s). The transformer can also be used to step up a lower voltage switching signal, so that the higher voltage required to drive the gate of at least one of the transistors is easily generated from a lower-voltage source. Such a single-side transformer coupled circuit is shown in U.S. Pat. No. 7,078,963 to Andersen, et al, in which a transformer is used to couple the control circuit to the positive side switching transistor.
However, such implementations typically require a relatively large number of passive components to complete the circuit, such as resistors, capacitors and/or snubbing/protection diodes to ensure that the gate of the transistor that is coupled to the transformer secondary is not damaged or improperly controlled, and that the transformer does not saturate due to a net DC magnetization current from duty cycles other than 50%.
Control of such a circuit is also typically complex. The required passive component values are dependent on the operating voltage ranges, static and Miller-effect transistor gate capacitance, which vary with operating conditions and environmental conditions such as temperature. In particular, the pulse width range that can be generated by such circuits is limited by the passive components, and if the output is to have a disabled state, such control must be provided by an additional circuit.
Therefore, it would be desirable to provide a transformer-isolated gate drive circuit that requires few or no passive components to achieve a wide pulse width range and has a robust characteristic over operating and environmental conditions. It would further be desirable to provide such a transformer-isolated gate drive circuit that can ensure operation of the transformer at substantially zero net magnetization current and that provides for the use of smaller transformers.